The present invention relates to a coiled resistance heating element of carbonaceous material. The "carbonaceous" herein referred to is an adjective generically modifying amorphous carbonaceous substance as well as crystalline graphitic substances. Therefore, the "coiled resistance heating element of carbonaceous material" herein referred to includes both those heating elements showing properties peculiar to carbon which are substantially composed of carbon and those showing peculiar properties of graphite which are substantially composed of graphite.
In a non-oxidizing atmosphere, carbonaceous materials show excellent heat resistance and corrosion resistance without being melted nor deformed. Also they have a good electric conductivity close to metals. With these properties, the carbonaceous materials are useful for the manufacture of heating elements for high-temperature electric or resistance furnaces, and have been heretofore used for heaters of laboratory furnaces such as Tammann furnaces and kryptol furnaces. Recently, as the semiconductor industry develops, such high-temperature electric furnaces have come to be utilized increasingly extensively as production equipment.
As high-temperature furnaces, are also known combustion furnaces, electric arc furnaces, plasma furnaces and electron beam furnaces. However, resistance furnaces are superior to any of those high-temperature furnaces in respects of uniformity of temperature distribution, temperature control accuracy and facility of furnace atmosphere control as well as in the aspect of environmental protection such as noise or exhaust gas control.
Here, some advantageous features of the carbonaceous materials as used in the form of resistance heating elements will be reviewed briefly. The carbonaceous materials do not melt under normal pressures with their sublimation temperatures as high as about 3,650.degree. C. and have extremely low vapor pressure in the order of 10.sup. 6 atm. at 2,200.degree. C. They show a high resistance to corrosive gasses. Their emissivity is as high as about 0.8. Unlike metallic materials, the carbonaceous materials do not soften at elevated temperatures, but their strength increases with temperature up to 2,500.degree. C. They have a moderate electric resistance. Especially, resistance of graphitic materials increases with temperature if exceeds about 500.degree. C. With low coefficient of thermal expansion, the carbonaceous materials have a high thermal shock resistance. Very high purity may be attained and they emit only a limited quantity of gasses even under high vacuum. Further, the carbonaceous materials are substantially inexpensive as compared with other materials of heating elements such as platinum, rhodium, wolfram, molybdenum, tantalum and silicon carbide.
However, unlike metallic and plastic materials carbonaceous materials have very low malleability and are extremely hard to work with precision to desired shapes. Therefore, in a known process for manufacturing heating elements, the carbonaceous materials are first molded into large-sized blocks and pieces cut out therefrom are subjected to cutting using a numerically controlled lathe or the like involving difficult and complicated operations. Especially, in tubular or planar heating elements of carbonaceous materials, notches are cut, for example, spirally or longitudinally to increase electric resistance, or they are worked otherwise specially. However, despite of such contrivance, the conventional heating elements of carbonaceous materials are not satisfactory in many respects. Not only difficulties are encountered in their working, but also they are massive. Since mechanical strength is decreased at notched portions, their structures, especially of those portions connecting terminals, require special consideration in design to prevent application thereto of bending, tensile and the like stresses caused by thermal expansion and contraction. Further, special care must be paid to handling of such heating elements of the prior art.
More recently, use of clothes or cords made of carbon fibers as heating elements having flexibility has been proposed. The flexibility permits such heating elements to be wound around furnace bodies and therefore inconveniences or disadvantages of the preceding heating elements of carbonaceous materials can be compensated for to some extent. However, since clothes or cords of carbon fiber have only a poor elasticity by nature, special retaining means must be provided to secure close contact onto furnace bodies. If such heating elements are bound tightly on a furnace to improve the closeness of contact, their durability will be decreased under stresses caused in use by thermal expansion and contraction. Further, the expense of carbon fibers adds to the costs of clothes or cords made thereof.
In respects of close contact on furnace bodies, relief of internal stresses of heating elements, ease of handling and mounting, conventional coiled heating elements of metal wires are desirable because they have a suitable elasticity and flexibility. However, it will be furthermore advantageous and desirable if coiled resistance heating elements of carbonaceous materials are developed. Nevertheless, such coiled heating elements of carbonaceous materials having a high strength have not been manufacturable by cutting of molded carbon blocks as prevalent in the prior art, so long as at least commercial-scale production is concerned.